


The Worst Case Scenario

by ReaderRose



Category: Undertale (Video Game)
Genre: Abuse, Alternate Universe, Alternate Universe - Swapfell, Angst, Character Death, Cussing, Dark, Darkfic, Destiny, EXP and LOVE (Undertale), Fellverse Awfulness, Gen, Heavy Angst, Human Experimentation, Identity Issues, Loss of Control, Loss of Identity, Memory Alteration, Memory Loss, Military, Neglect, Not Beta Read, Not Happy, Plot Twists, Programming, Sans-centric, Unethical Experimentation, Violence
Language: English
Status: Completed
Published: 2018-06-07
Updated: 2018-06-07
Packaged: 2019-05-19 09:08:25
Rating: Teen And Up Audiences
Warnings: Creator Chose Not To Use Archive Warnings, Graphic Depictions Of Violence, Major Character Death
Chapters: 2
Words: 3,507
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/14870882
Author URL: https://archiveofourown.org/users/ReaderRose/pseuds/ReaderRose
Summary: FELL AUMost would tell ya that the guiding role of this world is kill or be killed. But that's not really it.The real rule that Sans lives by is this:“if there's a possible worst case scenario, it's always gonna be the worst case scenario.”





	1. Chapter 1

**Author's Note:**

> Boy! Talk about fics that accidentally write themselves!
> 
> I wrote all of this this morning in a single go. Which is weird because lately stuff I WANT to write gets exactly 3 paragraphs in and then dies, but this fic just went from 0 to 3000 in no time.
> 
> Warnings for propaganda, indoctrination, memory issues, -fell bullshit, LV issues, violence, abuse, a lot of cursing, identity issues, paranoia, memory issues, and just a lot of darker themes
> 
> Also warnings for some potential continuity/tense issues. This started off as a different fic than it ended up as, but I won't have time today to do a lot of editting, and I'm going on a trip for a few days so there's a good chance I won't care enough to edit and post this if I wait until I get back.

Sans knows one principle beyond all others to be true in this world; that's how he survived as long as he has. 

Most will tell ya that the guiding rule of this world is kill or be killed. But that's a lie, definitely. Maybe you could even call it propaganda…if you knew a rep from the guard wasn't standing somewhere around breathing down your neck, which is why no one ever says that... but either way, it isn’t true. You can get through life without killing if you know the proper steps and loopholes and you’re willing to get away with living with less. And plenty of monsters out there don't want to kill, and most of them (not all of them, but certainly most of them) don't want to be killed, either. 

So it isn’t really a rule of the world. It’s just a rule of the rulers. (But again, you don't say that sort of thing out loud.)

But there is a real, hard and fast rule that Sans likes to live by:

**“if there's a worst case scenario, it's _always_ gonna be the worst case scenario.”**

Maybe it hasn't _always_ been accurate, but it's scored more frequently than “kill or be killed” on Sans's internal checklist, because after all, Sans? Has never had to kill. And yet here he is, not killed.

But worst case scenarios? He's seen ‘em come true every time. 

 

Take, for example, his “brother.” 

Now, Sans doesn't remember actually having a brother. It's kinda weird, right? This is another one of those things you never say out loud, but Sans's memory before a certain point is bad. Real bad.  Like, static and screaming and headaches kinda bad. 

So he just doesn't remember. 

But he’s never trusted Papyrus. Some guy shows up claiming to be your brother, in a world like this, and he wants something. You know he wants something. And when it happened, Sans didn't know what Papyrus wanted, but it's always the worst case scenario. Always. And even if it somehow wasn't, it would always pay to plan for it to be. 

So he avoided trusting him. Kept him at arm's length. Didn't let that shit about brotherly bonds hit too close to his heart, because damn if those kinds of emotional attachments aren’t the kind that weaken your soul and open up liabilities. Everyone knows that. Papyrus knew it too. Any guy that desperate to join up with the guard and be an arm of the crown that put them all here had to be pretty fucking aware of how dangerous attachments could be. 

 

So it made no sense that he was constantly offering up that attachment to Sans. Asking for hugs, or bringing up childhood memories that, as far as Sans knows, were totally made up. And Papyrus, for his part, didn't seem to buy them either. He always had this little… twitch about him, when he said something about their supposed shared childhood. And if Sans did engage and ask him a question to catch him off guard, suddenly the subject was changed entirely, usually to one of the, like, five established “memories” Papyrus always went on about.

There was one advantage to having a “brother,” though, and that was that no one messed with their stuff. Staying alive sometimes means concessions and tributes, but while Sans wasn't willing to handle all that hassle, Papyrus would always be the type to fight and go on and on about how none of it was fair, but how they could work things out. Papyrus was always asking for deals and truces and shit.

And that never went well. No one likes a moralizer.

And then someone would always end up dead. And it was never Sans, and it was never Papyrus, because for all his whining about a better world, Papyrus sure was quick to defend their stuff once the talking stopped or failed. 

Papyrus, you see, was a hypocrite. He talked a good game, but it was really just a game. Maybe the fighting was a game, too, because damn, Papyrus was constantly raking in the points at rates Sans had never even seen before.

Heh. But maybe one of those games got boring for him, while the other one didn’t. Took a while for Sans to realize just how much more quickly things were going from “WE CAN WORK THIS OUT!!!” to “YOU LEAVE ME NO CHOICE.” Eventually Papyrus was “left with no choice” about a minute into a confrontation, instead of after however long his prepared grandiose speeches took. Especially after Papyrus's official induction into the guard. He kinda just dropped the speeches after a while.

 

Actually, it’s… kinda funny. Those weren't the only speeches he used to make. There were ones like “EVEN THOUGH YOU DON'T REMEMBER, SANS, YOU'LL ALWAYS BE MY BROTHER!!!” or “YOU CAN ALWAYS RELY ON ME!! I'M THE RELIABILISTIST!” or “YOU’RE BETTER THAN YOU THINK YOU ARE.” or… he had a little speech for everything.

Sans hated them. 

He knew what manipulation felt like, and all those shouts of brotherhood and hope left him drowning in that feeling. Sure, Papyrus protected their shit. Yeah, he never tried to kill him. But at best it was a non-aggression pact between them. They didn't  _ talk _ talk. There was shit Papyrus was up to that he refused to ever say, and the secrets and the lies and the weirdly smarmy lines were just too much to even pretend it was an alliance. 

They lived under the same roof, and they vaguely looked alike, but they weren't brothers. 

 

The only thing that gave Sans pause, and it was a small thing… eventually Papyrus stopped with the speeches entirely. All of them. The “MERCY IS NOT WEAK!” ones to strangers they didn't know…and the other ones, too. 

Sans didn't  _ miss  _ them, but he… he thought about it a lot… kinda… kinda wondered why they stopped… 

And one day it hit him. Papyrus wasn't looking too good. Sans didn't care if the guy was sick, but that wasn't it. He had that… look. It was familiar enough for Sans to go “yeah I've seen that before” but not enough for him to recall exactly where, or on who. 

He was great at reading expressions, so that wasn't the problem. The problem was Papyrus never seemed to have an expression to read anymore. Any expression. He just looked kinda  _ weird _ and didn't say a whole lot. 

Oh, and he was fucking radiating LV.  That had been a slow creep, too. 

Sans realized a day too late what he was picking up on.

 

It was pretty fucking obvious, but for some reason he just… never connected the dots? 

It wasn't like the memories he couldn't call upon. It was more like he never expected his own brother to be someone who would succumb to LV.  Cuz, you know, Papyrus was the big damn hero who said a lot of stupid stuff but he didn’t like… he didn’t look for fights. And he didn’t… he didn’t seem like he enjoyed it? That much? LV was supposed to feel good and sometimes there was that… look… but… Sans always ignored it. Because he’d never cared that much about what Papyrus did. He seemed like he handled himself fine, so… 

So.

 

Yeah. 

 

He figured it out too late, because Papyrus fucking went ballistic. Feral monsters weren't  _ that  _ common, because once a monster started showing  _ those  _ signs, the Guard usually ended up taking them out. Or their families, if they had them, turned em in, or took care of them first. And that made sense because it was usually the families who would be the first to get destroyed if they let a feral monster linger in their own home. At that point, they didn't count as “family” anymore.

At that point, it really was kill or be killed…  

But Sans got lucky, which had literally never happened to him before. His so called brother lost his fucking mind, but he still didn't end up the victim of it. 

 

Instead, Papyrus tore up the palace. Made a huge scene, murdered a ton of guards, even tried to kill the queen. Can you fucking imagine? Because Sans sure couldn't when he heard about it. From those sappy speeches and tone deaf declarations, to a feral trying to rip chunks out of the Queen Of All Monsters with his bare fucking hands. 

Sans never saw it coming. 

 

It was this whole big affair for a while. A single freaking feral monster took out most of the higher command of the military and what they were left with was a fraction of what they started with. Some rookie hotshot got promoted to captain from the bottom almost overnight, because everyone above her was dead, and she was the one to stop him.  Eventually there would have to be a draft to get the numbers back up, but in the meantime it was a nightmare. Local gangs were going wild, and Sans had to watch his step like never before, because some people wanted to congratulate and thank him—he got free drinks at Grillby’s— and still others wanted revenge, either for their guard family members, or just for how much things sucked now that the Queen was cracking down on everything. 

And as much as he wanted to disavow any ties he had to Papyrus (and it’s not like he ever had that many ties to him in the first place), Sans just felt… empty about it. 

He was his brother… 

Or, well… 

He didn't know anymore. The worst case scenario he'd always expected hadn't come, and even now, he was just as alone as he always thought he was, no more, no less. 

He didn't know how to feel about it so he just didn't feel much about it at all. 


	2. Chapter 2

A month went by, or something like that. He wasn't keeping count. It happened and then days passed after that and Sans survived and things calmed down. The house got ransacked a few times.

Sans was never great at defending, just at getting out of people's ways. 

Which sucked. 

It sucked. It wasn't fair. That was his stuff. That was Papyrus's stuff. 

And it wasn't fucking fair. 

But since when did he ever think like that?

 

Maybe the worst case scenario had been that his brother rubbed off on him just enough to make him vaguely angry instead of just completely numb. He couldn't do cheerful, and he certainly couldn't do optimistic, but angry was well within his wheelhouse. 

Or maybe the worst case scenario was that he’d been wrong about his brother in the first place, but only noticed it when it was too late. 

 

Yeah, that seemed likely. 

He went with that. 

(Fuck.)

 

Sans never got back a box of ashes in the mail. 

He hadn't expected to, cuz, like, he went feral and tried to slaughter the queen. Papyrus’s dust was probably being kept for some ceremonial revenge or another and he didn't feel much about that. It was just dust. It's not like there was anything left for Sans to put the dust on anyway. Maybe Sans's own draft notice. That’d make for a funny funeral. He could honor him by follow in big brother's footsteps. (Was Papyrus the older brother? Truthfully, he didn’t even know. Papyrus’s childhood lies were inconsistent and now he’d never know the truth) 

Sans probably couldn't manage to reenact that ending part of Papyrus’s legacy, though when he got the official sealed letter in the mail, he sure thought about it.  
  


Months passed and Sans entered basic training, and it turned out he was better than he thought he was at fighting. The captain took a shine to him, too, saying they were both a lot alike. He didn't see how, but he also wasn't going to argue about it because he was just lucky she didn't want revenge for his brother ripping her eye out.

 

 

Time ticked by and Sans was alone for most of it. He kept his belongings light, his connections few, and suddenly it snuck up on him that the Royal Guard had become his whole life.

And it was so weird, because it felt so right. 

Like he was made for this, meant for this. Like there wasn't anything else worth worrying about. And that was simple and easy and he… he liked it. Rank and file and stepping in step. It felt exactly like where he belonged. 

Something nagged at him. He ignored it. Life went on. Sans was content.   
  


One day the Royal Scientist sent him a missive. Actually, it sounded like she sent him about a hundred, but he didn't go to the old house anymore, and usually he just stayed at the Castle. The queen needed a bodyguard, after all, and weirdly, they'd hit it off really well.

(The Queen said he was the most disciplined soldier she'd ever had, and nothing felt better than receiving such high praise.)

But the scientist was frantic when he finally called her back. She wouldn't say what she wanted, only that he needed to come in. She technically ranked above him, and an order was an order.  He couldn't say he was happy about it, though. Uncertainty made him nervous and reminded him of times he'd decided he'd leave forgotten, along with the things he really had lost from his mind. 

His nerves only worsened when he arrived at the lab. He wanted to run. Badly. But he didn't. He was done with running. He was a guard now.

 

The doctor was disheveled and the inside of her lab was even worse. Briefly, he admonished about how she'd let it get to such a state…  before remembering he'd never been here before. She didn't seem to catch it, or even hear what he was saying, all stutters and apologies and explanations. She didn't act the way he thought a royal scientist should— she wasn’t regal, or detached, or cold or— and the steady stream of “I'm so, so sorry” struck him harder for its strangeness than it did for the fact that he didn't know her at all, or why she would be apologizing to him at all. 

That went on for a bit, like they were speaking different languages. And then she went into the back room. 

 

 

And then she walked back out. 

And someone else walked back out with her. Behind her, head down, a shuffling gait. The way a prisoner walks to their execution, and they even had the hood for it, and the shackles, and the chain, right there around the neck.

 

It was Papyrus.

 

At least, she  _ said  _ it was Papyrus. And sure, it looked a bit like Papyrus.

But Papyrus was dead. 

 

Or at least… Sans hadn't asked questions because,  _ fuck, _ he'd never wanted to know the answers! Either Papyrus died when they stopped him, or they took him alive and didn't keep him that way. Those were the only two options. The worst options, right? Because… because  _ fuck!  _ They wouldn't keep him alive??? They wouldn't keep him alive and never mention it! They wouldn't return him with an apology after seven fucking months of _ nothing! _

_ “what the fuck is wrong with him!?” _ is the question that managed to break through instead.

 

Papyrus flinched, but otherwise just started, blank and empty. He didn't look feral, but wow… wow, he didn't look right. He didn’t look right at all, and Sans couldn’t even exactly say what it was that was different, only that it was _wrong._

Dr. Undyne stumbled on her words. Dodged the question. Dodged questions Sans hadn’t even thought to ask. Explained that she was working on a project involving ferals and that she “cured” him. He knew a liar when he saw one, but at least some of that seemed to be true. (But she was leaving out _something)_

But Papyrus didn't look fucking cured. And no one had ever come back from that before. And Undyne was staring at  _ him  _ in a way that made him feel all wrong and uncomfortable. He couldn't tell if she thought he was her next experiment, or a pet she wanted to give a hug and feed a little  biscuit. And that's definitely how she was treating Papyrus, all chained up like a dog, led around with that protective sort of gesturing that made no fucking sense, considering what he was and what he'd done. 

After she explained, Sans didn't feel like he knew any more than what he started with.

“what do you want me to do with him?”

“...T-take him home?”

 

There was no home. There was nowhere to take him. What, was he going to take him to the palace? Ha. Sure. That would go well. 

 

“give me a few days to figure this out,” Sans said, and left. It ended up being something like weeks. It didn't feel real enough to be a priority. He worked it out, though. Got reassigned to the middle of nowhere out in Snowdin. Took “Papyrus” off Undyne's hands and practically dragged him back to the backwater town. 

All he felt was agitation, being away from his primary duties. 

 

Papyrus was silent the entire time. 

His LV was bottomed out to 1. So were all his other stats. Undyne explained it but all Sans heard was  ~~_static. _ ~~

 

Most of Snowdin had heard about the feral that attacked the queen. Exactly none of Snowdin know that that feral was alive and in their midst. Sans saw no need to explain their move to the town, and the town soon learned to never expect an explanation out of of a skeleton. One would just insult you, and the other wouldn’t say anything at all.

 

When Papyrus saw Sans in uniform for the first time, it almost looked like he almost had a thought about it. It was a split second, but for that second he looked the most alive he had in years. Sans laughed. Yeah, he'd never thought he would be in the guard either. He kind of got why Papyrus always wanted to join even back when he was still all love-and-peace-and-protection. Papyrus always said it felt like he had to, and Sans never got that, but he knew he would never quit now that he was in. It was where he belonged. 

And if it wasn't for Papyrus's actions, he never would have found that sense of belonging. That rightness.

Seems like his brother was good for something after all!  
  


Things fell into an awkwardly familiar pattern. Sans mostly just ignored his brother, and his alleged brother mostly pretended he was still as dead as Sans once assumed he should be. But there were no speeches, and no defenses, and no hugs and no talking. 

At first it was fine. 

 

Then one day it just…. wasn't. 

Sans snapped. Lost his temper. Nothing triggered it. It just happened. Sans screamed at him. Cursed him. Questioned him. Hated him. 

And Papyrus stared back and said absolutely nothing. 

Sans did his best to look him straight in the eyes, wanting to capture every flinch and blink to try to decipher what the hell was going on, any of it, because something was ducking wrong here and Sans couldn't even begin to understand what it was. 

(And it was easy to blame papyrus when he looked so much like—)

“remember when you used to say i could rely on you? you never once showed that that was true.”

A twitch, but nothing said. Just empty sockets and what almost passed for sadness. 

“well???”

Papyrus whispered. “I DON'T REMEMBER.”

If Sans didn't  _ need  _ his anger so much, he would have felt it lapse at that. It certainly took some of the wind of of his sails, but he pushed onward instead, anyway. 

“so, what, you don't remember anything, now?? what about all those sweet little memories you used to claim we had that never happened? what about all those promises?? what about what you did to queen toriel!?!?”

“I REMEMBER THAT YOU'RE MY BROTHER.” Sans would have choked, if he were a lesser person. But he wasn’t, and those words didn’t affect him. “BUT THAT'S IT. I DON'T EVEN KNOW WHAT THAT MEANS…”

Sans huffed. He didn't understand any of this.

“it means you do what i tell you and get out of my way.”

Papyrus nodded.  “OKAY.”

 

And that was the end of that. 

 

* * *

 

 

Sans doesn't understand a lot of things. What he does understand, is if there’s a possible worst case scenario, it’s probably going to be the worst case scenario.

Maybe the worst case scenario here was fucking up his relationship with his brother, twice. 

But with how little Sans really knows about himself and his brother and the world, besides that one simple rule, it’s always possible the worst case scenario was something he hasn't thought of yet. 

 

And the worst case scenario is that he’ll never know it in time.

**Author's Note:**

> surprise its one of those underswap-underfell mashup aus. 
> 
> Advantage to not labeling the specific AU for the sake of it being a mild twist: not having to figure out what that AU is calling itself these days.
> 
> This isn't my headcanon for this AU, actually, but I was already going for a certain style of underfell and thought "wow this would make so much more sense with swapfell instead"
> 
> Sans's narration style changing is partically because of that, but also because he REALLY took to the guard.
> 
> EDIT: LOOK!!! FollowerofMercy made fanart!!! http://followerofmercy.tumblr.com/post/175766069784


End file.
